1. Field of the Invention
The present invention relates to a spark arrester of an internal combustion engine for portable working machines.
2. Description of the Related Art
A spark arresting device or arrester used in an engine-powered portable working machine, such as a weeding machine or a chain saw, is disclosed in, for example, Japanese Utility Model Publication No. SHO 60-33290. The disclosed spark arrester has a wire screen attached to the outlet end of a tail pipe for arresting or catching sparks in exhaust gases to thereby prevent the sparks from escaping into the atmosphere.
FIG. 6 of the accompanying drawings shows a spark arrester using a similar wire screen. As shown in FIG. 6, the spark arrester 100 is attached to a tail pipe 102 of a muffler 101 and includes a stay 104 fitted around the tail pipe 102, a flared pipe 105 attached by clenching to the stay 104, and a wire screen 106 attached by clenching to a wide outer end 105a of the flared pipe 105.
Exhaust gases from an internal combustion engine 107 pass successively through a first expansion chamber 108 and a second expansion chamber 109 of the muffler 101 and flow into the tail pipe 102. Then, sparks in the exhaust gases are arrested or caught by the wire screen 106 and, thereafter, the exhaust gases are released into the atmosphere.
The exhaust gases, as they are discharged from the tail pipe 102 through the wire screen 106 into the atmosphere, form a stream of hot gases having a certain temperature distribution which is composed of a relatively high-temperature inner area or section S1 (indicated by hatching for clarity) and a relatively low-temperature outer area or section S2. The inner section S1 of the hot gas stream flowing out from the wire screen 106 has a length L1 smaller than the length L2 of the outer section S2.
The length of such hot exhaust gas stream including the two temperature sections S1 and S2 should preferably be minimized because a longer hot exhaust gas stream is likely to affect or damage the user and surroundings of an engine-powered working machine, such as a chain saw, on which the spark arrester is attached.